1. Field of the Invention
The present invention is directed to coupling agents. More particularly, the present invention relates to blends of maleated polyolefins for use as coupling agents in polymeric matrices.
2. Description of Related Art
Performance requirements in automobile and other consumer industries are becoming more stringent owing to legislation, and products must be designed to meet those requirements. One example is a good flame retardant rating (V0) in both automobile and non-automobile industries. Products with improved performance conforming to set standards are not only necessary for the maintenance of existing business but are also essential in targeting new applications business growth. Thus, new product development often arises out of necessity.
Polymers comprising maleic anhydride grafted onto polypropylene (PP-g-MA) are widely used as coupling agents and compatibilizers in glass-filled polypropylene, talc-filled polypropylene, nylon/polypropylene blends, wood/plastic composites, and nano-composites in a large number of applications in automobiles and other consumer products. This has helped polypropylenes make an inroad into some very demanding automobile structural applications. These coupling agents are mostly based on homopolymer polypropylene where a high melting point along with high strength and stiffness properties are key requirements.
Polypropylene-g-MA is currently used as a coupling agent in polypropylene based wire harness for automobile applications and wire jackets for home appliance applications. Now, these applications are required to meet V0 flame retardant rating. Higher filler loadings are required to achieve this target, but polypropylene homopolymer-based products become stiffer and lose their flexibility when this is done. Random and impact polypropylene copolymers are being looked at as matrices for the development of these products. Polypropylene homopolymer-based PP-g-MA products are not appropriate for such applications, as they negatively affect the flexibility of the products.
The most common production method for polypropylene-g-MA products is via reactive extrusion using organic peroxide under controlled conditions. Owing to the high reaction temperature (above the melting point of polypropylene) requirements, the polypropylene chain undergoes scission along with reaction with maleic anhydride and a higher melt flow rate results. Further, the maleic anhydride content is generally low. Attempts to increase the maleic anhydride incorporation results in very high melt flow, as well as discoloration owing to polypropylene degradation and maleic anhydride oxidation. At some point, as the maleic anhydride is increased, the melt flow rate (MFR) can become so high (i.e., the molecular weight becomes very low) that the coupling agent tends to lose effectiveness.
As noted above, polypropylene impact or random copolymers can be used as the base material for maleic anhydride grafting to produce very low stiffness coupling agents. Unfortunately, at elevated reaction temperatures, any polyethylene segments of such copolymers often undergo cross-linking reactions along with the maleic anhydride grafting reaction, thereby producing a higher fraction of gel content. This results in a lower melt flow rate (MFR), which may inhibit dispersion of coupling agent in the final blend formulations.
A solid state grafting process is another production method for this type of product. Here, the reaction is carried out below the melting point of a precursor, such as a polypropylene homopolymer, random copolymer, impact copolymer, etc. The precursor degradation in this process is less than in reactive extrusion processes and, hence, the product MFR is more controllable. On the other hand, to minimize any cross-linking reaction, the reaction temperature is kept low, whereby the maleic anhydride incorporation is not very high. To compensate for this and achieve higher maleic anhydride incorporation, higher amounts of maleic anhydride and peroxide can be used, but then unreacted MA has to be removed. This method also results in an undesirably higher amount of free maleic anhydride in the product.
U.S. Pat. No. 3,414,551 discloses a process for reacting maleic anhydride with a crystalline polymer which comprises fluidizing the polymer in particulate form in contact with vapors of maleic anhydride and an organic peroxygen compound in an atmosphere of inert gas at a temperature of from about 80° C. up to a temperature about 10° below the melting point of the polymer in the absence of a solvent for the polymer for a time to cause chemical reaction of the polymer with from about 0.2 to 10% of maleic anhydride, based on the weight of the polymer.
EP 0 519 341 B 1 discloses a uniformly grafted particulate polyolefin material formed by the free radical-initiated grafting of at least one non-homopolymerizing vinyl monomer at free radical sites on a particulate olefin polymer material having (a) a pore volume fraction of at least about 0.07 wherein more than 40% of the pores have a diameter larger than 1 micron; and (b) a weight average diameter in the range of about 0.4 to 7 mm. The amount of vinyl monomer grafted to the as-polymerized particulate olefin polymer material is about 0.1 to 10%, preferably about 0.3 to 5.0%, of the total weight of the grafted olefin polymer product, and the grafted vinyl monomer is uniformly distributed throughout the particles of the olefin polymer material.
A need exists for the development of soft coupling agents having low flex modulus (e.g., 100-1000 MPA), high maleic anhydride content (greater than about 1%), and having an MFR high enough to allow ready dispersion in final compositions, but not so high as to lose effectiveness as a coupling agent. It is also evident that a need exists to enable independent control of maleic anhydride content and melt flow rate.